Most high voltage bushings have one dielectric tube external to a building or container. Feed-through bushings that exit from a building often have a second dielectric tube internal to the building. The tubes are usually joined together by a coupling where the bushing passes through the building. Often the tubes are sealed and pressurized with air, nitrogen (N2) or sulfur hexaflouride (SF6) to increase the withstand voltage between a high voltage center conductor mounted in the bushing and the ground potential at the pont where they penetrate the building. The dielectric tubes are most commonly made out of fiberglass covered with silicon rubber materials.
There are many phenomena that limit the voltage and current that a particular bushing can handle. Some of these limits are flashover (internal or external), internal heating (current), and corona formation on the surface of the dielectric tubes. Corona is very hot plasma, and over the course of time, it can damage the surface or destroy the dielectric tube. The flashover and corona formation voltage on the portion of the bushing that is outside the building and exposed to the elements is dependent upon the environmental conditions. Wet conditions, typically from rain or condensation, are the worst case. In wet conditions, the voltages at which the deleterious phenomena occur are lower. For example, at a certain voltage under dry conditions, the bushing may be completely corona free, but corona can form on the surface of the external dielectric tube when the bushing becomes wet, as for example, when exposed to rain. The energy dissipated by corona is proportional to frequency and the potential for damage is exacerbated at radio frequency. For that reason, one of the critical design criteria for RF (VLF/LF) high voltage bushings is that there be no stationary corona on the surface of the dielectric tubes under spray wet conditions. Obviously, the portion of the bushing that is located inside of the building does not become wet and does not have to satisfy this criterion.
The formation of corona under spray-wet conditions is a function of the electric field strength on the surface of the dielectric tubes that come in contact with water drops. Consequently, one aspect of designing a high voltage bushing is to minimize the surface electric field on the surface of the external dielectric tube. For a simple bushing with a cylindrical center conductor and a cylindrical tube, the surface electric field on the external tube is smallest near the tip of the bushing, and is a maximum at the region of the bushing where the ground potential is located. The average electric field along the surface of the dielectric tube is equal to the voltage divided by the length of the tube. However, the maximum electric field at the grounded region of the bushing can be 5 to 10 times (or more) greater than the average electric field, depending upon the relative size of the hole through the building through which the bushing penetrates, and the diameter of the center conductor. In a typical composite bushing, there is a ring-shaped region, referenced as the “triple point,” where three structures made of three different materials, the silicon rubber shed, the aluminum center flange, and the dielectric (fiberglass) tube, are coterminous. Typically, the “triple point” is the first region of the bushing likely to go into corona at a threshold electrical field intensity. It is common practice to shield the “triple point” region and the internal portion of the bushing where the center conductor passes through the conducting wall with an internal ground shield to moderate the electric field. Without the ground shield, the electric field would not be moderated at all, whereupon the external surface electric field would be heavily concentrated on the dielectric tube near the triple point region.
An optimum bushing design would have a uniform electric field along the surface of the external dielectric. Such an electric field would be equal to the voltage across the busing divided by the length of the dielectric tube. One way to achieve a uniform electric field is to have one or more floating conducting tubes of varying length to cause the fields to distribute nearly linearly over the surface of the dielectric tube. This approach has been used for 60 Hz high voltage bushings, but the methods used to insulate such floating tubes have associated technical difficulties and expense. For example, heating of the dielectric used to support the floating tubes in position can limit the temperature rise and hence the current limit of the busing. Perhaps more importantly, the floating tubes significantly increase the capacitance of the bushing, which is deleterious for RF antenna applications.
A need therefore exists for a high voltage bushing that overcomes the aforesaid technical difficulties and reduces the electric field in the region of the “triple point” to prevent the formation of corona when the bushing is wet.